LOWER HELDERBERG ROCKS. 239 



Rhynclionella cainpbellana ( n. ».). 



Plate XLIII. Fia. 2 a - k. 



Shell longitudinally oval, ovate or oblong, laterally compressed, two-thirds 

 as broad as long, length and height about equal. Dorsal valve the larger, 

 elevated near the front into a broad undefined mesial fold, declining 

 near the beak and ctu-ving abruptly at the sides : beak incurved. Ven- 

 tral valve compressed, abruptly deflected towards the opposite valve 

 at the lateral margins, depressed into a broad rounded sinus which 

 occupies almost the entire breadth of the narrow front : front margin 

 curving upward, and extended into a subtriangular prolongation. 



Surface marked by twenty-two to twenty-four simple rounded -subangular 

 plications, five or six of which are elevated on the mesial fold, and four 

 or five occupy the sinus of the ventral valve. Fine zigzag lines of growth 

 are seen on the front of the shell, near the junction of the valves. 



This species bears much resemblance to the R. nohilis in many of its characters, 

 but the depressed ventral valve and straight sides are pretty constant and distin- 

 guishing features. The casts of the two species show still more conspicuous dif- 

 ferences, as may be seen in comparing figures 2/ and 3 /. 



Fig. 2 a - e. Ventral, dorsal, and front views of young specimens. 



Fig. 2 /, h. Dorsal and profile views of an individual of full size. 



Fig. 2 g, i. Ventral and front views of an individual of full size, which is broader in the 

 middle than usual. 



Fig. 2 k. Enlargement of several plications of the front of the ventral valve. 

 Fig. 2/. A cast of the ventral valve [ the letter / should he I]. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group, and in the succeeding " Scutella limestone " of the same group : Helderberg 

 mountains, Albany county. 



